The induction of metamorphosis in Aurelia will be investigated in detail. The thyroxin-induced cytological changes during the pre-segmentation and early segmentation period of strobilation will be followed at the light microscope and electron microscope levels. Specifically to be determined is whether increased mitotic activity occurs during this period and if so, which cell types are dividing. Also, the nerves in the polyps during the early stages of strobilation will be investigated to determine whether the number of nerves or of neurosecretory granules increases. In general, any cytological differences in the thyroxin-induced organisms will be noted and recorded. These studies will hopefully improve our understanding of the action of thyroxin at the cellular level on developmental processes of higher organisms as well as on jellyfish.